Of the many different types of commercial plastics, both engineering and commodity types, linear polyethylenes (PE) hold a special place. Despite high resistance to chemicals, high electrical resistance, and the ability to be easily molded by numerous methods, this plastic is still the least costly resin available commercially. Certain grades of PE also offer strong resistance to diffusion by moisture and one grade in particular, called ultrahigh molecular weight (UHMW) PE, is considered tougher than any other commercial plastic, even more abrasion resistant than steel. UHMW PE is useful for bullet-proof vests, long-lived gears and even human bone replacement.
One characteristic of PE that is highly valued during processing is its melt elasticity. This property, also called “shear-thinning”, is determined to a limited extent by the breadth of the molecular weight distribution and the amount of long chain branching. Thus, certain grades made from chromium and metallocene catalysts exhibit some shear-thinning character, while other PE grades, particularly those from Ziegler-Natta catalysts, display little. The degree of melt elasticity in PE is important because it determines much of the processing behavior of the molten polymers. Polymers with a high degree of elasticity, or shear-thinning character, flow easily under stress at high shear rates, but resist flow when allowed to stand under low or zero strain. In blow molding for example, the parison is extruded easily from the die under pressure, but once the molten tube has exited the die and there is no flow, it resists being pulled out of shape by gravity. Likewise, melt elasticity influences (1) swell during blow molding, (2) bubble stability, orientation, melt fracture, and extensional viscosity in film blowing, (3) melt strength in geomembrane resins, (4) sag during large diameter pipe extrusion, and even (5) various mechanical properties in the finished articles.
One method of improving the elasticity of the molten PE polymer, as well as some of the physical characteristics, is to blend in fillers such as calcium carbonate, talc, clay nano-composites, carbon black, as well as carbon and glass fibers. In general these materials do increase the shear thinning character of the polymer, but to make a significant difference so much filler is required that it becomes impractical for other reasons. Although in some applications a higher conductivity would be desired, these fillers usually contribute little or nothing to the conductivity of the polymer composite.
Another advantage of highly shear thinning resins is their behavior during a fire. While not actually fire retardant, such materials do tend to hold together better during combustion, with less running or spattering that can spread the fire. Although polymers with high levels of long chain branching can be shear thinning, they also tend to have a high Ea, that is, a high activation energy for melt viscosity. This means than the melt viscosity decreases sharply with increasing temperature. Nonpolymeric fillers, on the other hand, tend to lower Ea, meaning that the viscosity contribution from the filler is less affected by temperature, allowing the low-shear viscosity to remain high even during a fire.
Conductivity is another feature that is often desirable in plastics. PE and polypropylene (PP) are not naturally conductive, and their use in chemical storage, fuel tanks, drums and other applications often requires grounding. This is usually accomplished not by modifying the resin itself, but by the addition of some form of conductive paint or metal attachment, which usually adds to the difficulty and cost of production. The usual fillers are not conductive at all; metal powders are the single exception, and an impractically large amount is required to make the final article conductive.
It would be desirable therefore to be able to produce polymer composites having high shear thinning character (high melt elasticity) which are also highly conductive.